The normal endothelial lining is highly important in rendering the luminal surface of the vascular system non-thrombogenic by partitioning the coagulant triggers of the underlying tissue from the procoagulants of plasma. Recent studies indicate that the endothelial cell also synthesizes and secretes plasminogen activator(PA). Presumably, imbalances in this physiological process could contribute to the development of thrombosis. The proposed research will focus on 1) the isolation and purification of a canine, vascular-derived PA; 2) a study of the mechanism of plasminogen activation by PA and its interaction with other plasma and vascular-derived components; 3) the production of antiserum to the PA for use in a) quantitating the distribution of this activator in this species and b) in in vivo studies of the physiological factors affecting its release into the circulation; 4) beginning initial studies on the influence of aging on these processes; and 5) determining the relevance of these findings to the human fibrinolytic system. The methods will emphasize protein purifications, enzymatic and immuno-assays, electrophoresis, immunochemistry, isotopic tracers and endothelial cell culture. The integration of both physiological and biochemical results should permit the construction of a model of the in vivo fibrinolytic activation process as it occurs at the blood-vessel wall interface and have clinical relevance in regard to thrombolytic therapy.